1) Field of the Invention
The present invention relates to a battery-voltage monitoring integrated-circuit and a battery-voltage monitoring system for monitoring a voltage of a secondary (rechargeable) battery, such as a lithium-ion rechargeable battery.
2) Description of the Related Art
Compared to other rechargeable batteries, a lithium-ion battery has the following advantages, namely, it produces high-energy density (high output density), gives a high battery voltage (4.2 V) per battery cell, has a high charging-discharging energy efficiency, has no memory effect, etc. Consequently, lithium-ion batteries are being widely used in compact gadgets such as, mobile phones, video cameras, notebook computers, etc.
Because of these advantages, lithium-ion batteries are expected to replace the conventionally used nickel-metal-hydride batteries employed in hybrid vehicles, motor driving devices, etc.
However, the property of high-energy density of a lithium-ion battery can make it potentially dangerous should a short-circuit occur within it due to any reason, since the energy is released all at once. Therefore, to avoid any mishap, it is usual practice to take precautionary measures such as using a heat-stable material, avoiding over-charging or over-discharging the battery, or even if the battery is over-charged or over-discharged, taking care not to allow the temperature of the battery to rise and causing it to ignite.
As a measure against over-charging or over-discharging, a method for monitoring the voltage of the lithium-ion battery is employed. In other words, the lithium-ion battery, which usually consists of a plurality of serially-connected battery cells that output the voltage required for the gadget in which the battery is used, is monitored, and when any of the battery cells is over-discharged or over-charged, an error signal is output, causing a forced stopping of charging or discharging (see, for example, Japanese Patent Laid-Open Publication No. 2001-224138 and Japanese Patent Laid-Open Publication No. 2001-85067).
An integrated-circuit for battery voltage monitoring may be used for monitoring the battery cells of the lithium-ion battery and outputting a detection signal when any battery cell shows signs of over-charging or over-discharging, thus providing a system for monitoring battery voltage that performs efficient voltage monitoring.
However, hybrid vehicles and motor driving device require high voltage, necessitating serially connecting an appropriate number of the lithium-ion batteries to obtain the required voltage. If a single battery-voltage monitoring integrated-circuit is to be used in such a setup, the integrated-circuit needs to be constructed from an element that can withstand high voltage, since it has to withstand the voltage from all the battery cells, and the surface area of the chip has to be considerably large, pushing up the cost.
Further, since the voltage required by different gadgets are different, the battery-voltage monitoring integrated-circuit, built for gadgets that require high voltage levels, will prove to be an overkill for the gadgets that need less voltage, again pushing up the cost.
To overcome this drawback, the battery-voltage monitoring integrated-circuit is built for handling a specific number of lithium-ion batteries. If the number of cells in the serially-connected battery cell group being monitored exceeds the range, as many battery-voltage monitoring integrated-circuits as are required may be connected serially, stacking them one on top of another. This method makes the battery-voltage monitoring integrated-circuit cost-effective.
However, even in the method using stacked battery voltage monitoring circuits, since only a single control circuit controls charging and discharging based on the detection result, how the detection result of each of the battery-voltage monitoring integrated-circuits is to be transmitted to the single control circuit poses a problem.
A method of insulating the detection result from each of the battery-voltage monitoring integrated-circuit by a photo coupler, etc. and transmitting to the control circuit by changing the voltage to the ground voltage may be adopted. However, this method will again push up the cost since a photo coupler needs to be provided for each battery-voltage monitoring integrated-circuit.
Other methods like providing a circuit that integrates the detection results from all the stacked battery-voltage monitoring integrated-circuits, or providing a control circuit having as many number of input terminals as there are battery-voltage monitoring integrated-circuits, etc. will not only make the circuitry complicated, but will also push up the cost.